linux-kernel - Re: [PATCH 16/31] gpu: nova-core: Hopper/Blackwell: add FSP message infrastructure

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Message-ID: <37a85b2e-603c-44ad-a346-783842e9c829@nvidia.com>
Date: Fri, 5 Dec 2025 11:47:28 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: John Hubbard <jhubbard@...dia.com>, Danilo Krummrich <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 Alistair Popple <apopple@...dia.com>, Edwin Peer <epeer@...dia.com>,
 Zhi Wang <zhiw@...dia.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 nouveau@...ts.freedesktop.org, rust-for-linux@...r.kernel.org,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/31] gpu: nova-core: Hopper/Blackwell: add FSP message
 infrastructure



On 12/3/2025 12:59 AM, John Hubbard wrote:
> Add the FSP messaging infrastructure needed for Chain of Trust
> communication on Hopper/Blackwell GPUs.
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  drivers/gpu/nova-core/falcon/fsp.rs | 77 +++++++++++++++++++++++++++++
>  drivers/gpu/nova-core/regs.rs       | 48 ++++++++++++++++++
>  2 files changed, 125 insertions(+)
> 
> diff --git a/drivers/gpu/nova-core/falcon/fsp.rs b/drivers/gpu/nova-core/falcon/fsp.rs
> index 9e796e82e556..0e8522b1171d 100644
> --- a/drivers/gpu/nova-core/falcon/fsp.rs
> +++ b/drivers/gpu/nova-core/falcon/fsp.rs
> @@ -87,4 +87,81 @@ pub(crate) fn read_emem(&self, bar: &Bar0, offset: u32, data: &mut [u8]) -> Resu
>  
>          Ok(())
>      }
> +
> +    /// Poll FSP for incoming data.
> +    ///
> +    /// Returns the size of available data in bytes, or 0 if no data is available.
> +    ///
> +    /// The FSP message queue is not circular - pointers are reset to 0 after each
> +    /// message exchange, so `tail >= head` is always true when data is present.
> +    #[allow(dead_code)]
> +    pub(crate) fn poll_msgq(&self, bar: &Bar0) -> u32 {
> +        let head = regs::NV_PFSP_MSGQ_HEAD::read(bar).address();
> +        let tail = regs::NV_PFSP_MSGQ_TAIL::read(bar).address();
> +
> +        if head == tail {
> +            return 0;
> +        }
> +
> +        // TAIL points at last DWORD written, so add 4 to get total size
> +        (tail - head) + 4
> +    }
> +

Can we harden against integer underflow here? Even if tail is always >= head in
normal operation, I think we should prepare for underflow due to misbehaving HW
etc to avoid producing a massive integer, how about:

    tail.saturating_sub(head) + 4

Thanks.